1. Field of the Invention
The present invention relates to a communication terminal device that is preferably used for a radio-frequency identification device (RFID) system and a near field communication system.
2. Description of the Related Art
In an RFID system in a high frequency (HF) range and a near field communication (NFC) system, an antenna for communication is mounted on each communication terminal device such as a cellular phone terminal in order to perform communication between the communication terminal device and a reader/writer or perform communication between the communication terminal devices. Japanese Patent No. 4687832 discloses an antenna that is provided on a communication terminal device as described above.
As disclosed in Japanese Patent No. 4687832, an opening and a slit are provided on a planar conductor to which a power feeding coil comes close such that current induced by a magnetic field generated in the power feeding coil flows along end edges thereof (so that the magnetic field is not cancelled).
In a mobile communication terminal device such as a smart phone, various components are mounted with high density and a metal body (hereinafter, “rear metal body”) having a large area, such as a metal chassis and a shield plate, tends to be arranged in the vicinity of the planar conductor and the power feeding coil. With this configuration, the rear metal body acts so as to cancel electric current flowing through the power feeding coil and the induced current flowing through the planar conductor in some cases. This results in the lowering of antenna radiation characteristics by the power feeding coil and the planar conductor in some cases.
When a magnetic body sheet is interposed between the rear metal body and the power feeding coil and a thickness dimension and an area of the magnetic body sheet are increased, the power feeding coil and the planar conductor are magnetically shielded from the rear metal body. Accordingly, the phenomenon that the current flowing through the power feeding coil and the current flowing through the planar conductor are cancelled by the current induced in the rear metal body can be suppressed. However, the magnetic body sheet is expensive, and an increase thereof in thickness and size leads increase in cost. Further, the increase thereof in the thickness and the size can be a factor inhibiting reduction in size and increase in density of the mobile communication terminal device.